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J.RADIOANAL.NUCL.CHEM.,LETTERS 187 (5) 375-383 (1994)
PHOTOCHEMICAL REDUCTION OF URANYL ION WITH TRIETHYLAMINE
M.S. Sidhu, K.B. Kohli, P.V.K. Bhatia, S.S. Sandhu
Department of Chemistry, Guru Nanak Dev University, Amritsar - 143 005, India
Received 24 January 1994 Accepted I February 1994
Uranyl ion is photochemically reduced to uranium(IV) ~n the presence of triethyl- amine and triethylamine is oxidized to secondary amine and acetaldehyde. On the basis of product analysis, temperature independent quantum yields for uranium(IV) formation and abnormal Stern-Volmer plots rule out the simple collisional photo- chemical annihilation of excited uranyl ion with triethylamine. Static annihila- tion has a significant contribution in addition to dynamic annihilation.
INTRODUCTION
A variety of organic compounds efficiently annihilate
electronically excited uranyl ion either through colli-
sional electron transfer or a hydrogen atom abstraction
mechanism I-5. Linear Stern-Volmer plots for guenching
the excited uranyl ion with phosphorus(III)/sulfur(II)
substrates involves collisional electron transfer to ex-
cited uranyl ion followed by oxo-oxygen atom transfer to
375 Elsevier Science S. A., Lausanne Akaddmiai Kiad6, Budapest
SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION
phosphorus(III)/sulfur(II) atoms, respectively, facili- 6-9
tated by d~-p~ interactions The present investiga-
tion involves the photochemical reduction of uranyl ion
with the nitrogen donor triethylamine. Positive devia-
tions from the straight line for Stern-Volmer plots
have been rationalized through dynamic and static 10
ground state complex formation
EXPERIMENTAL
Uranyl acetate, triethylamine, sulfuric acid and ace-
tone were A.R. Grade chemicals used as supplied. Deion-
ized doubly distilled water-acetone (30:70 v/v) was used
as reaction medium. Photolysis was carried out in a
pyrex glass photochemical reactor using 125 W medium
pressure mercury lamp (Hanovia Lamps). Quantum yields
were determined using a potassium ferrioxalate actinom-
eter (Table 1). The relative intensities of uranyl ion
luminescence at 506 nm were measured with a Perkin-
Elmer-LS-2B filter fluorimeter with excitation at ~ 345
nm. Electronic absorption spectra of photolyzed solu-
tions were recorded with a 240-UV Shimadzu recording
spectrophotometer, p-Nitroaniline and m-nitroaniline
fail to show a photochemical reaction but quench the
uranyl ion luminescence.
RESULTS AND DISCUSSION
Uranyl ion in acidic medium absorbs light in the
visible region (lmax = 420 nm). Addition of increasing
amounts of triethylamine (0.00-0.007M) does not shift
but gives a small depression in the absorbance at max
376
SIDHU et ~.: PHOTOCHEMICAL REDUCTION OF URANYL ION
TABLE 1
Quantum yield and Stern-Volmer constants for uranium(IV) formation in photochemical reduction of
uranyl ion with triethylamine, [H +] = 0.2M at 30• ~
Sr. No. [N(C2H5)3], M [UO22+], M ~U(IV) Ksv, M-I
I 0.05 0.010 0.15
2 0.10 0.010 0.17
3 0.15 0.010 0.20
4 0.20 0.010 0.23
5 0.25 0.010 0.26
6 0.10 0.011 0.18
7 0.10 0.012 0.18
8 0.10 0.013 0.18
m - Nitroaniline
p - Nitroaniline
114.28
88.88
66.66
420 nm. However, further increase in triethylamine con-
centration (0.007"0.010M) leads to an enhancement in the
absorbance (Fig. I). Preferential protonation of tri-
ethylamine prevents its interaction with uranyl ion but
excess triethylamine interacts with uranyl ion because
of hard-hard interactions in the ground state and makes
the environment 0f the uranyl ion more rigid 11
Optical excitation of uranyl ion with visible light
renders the uranium-oxygen bond weaker and longer, since
an electron hops from the highest occupied molecular Or-
bital consisting of uranium 5f/6d and oxygen 2p atomic
orbitals to the lowest unoccupied molecular orbital con-
sisting of only uranium 5f orbital I'2'12'13 Triethyl-
amine is transparent to visible light in a pyrex glass
photochemical reactor. Consequently, absorbance of light
377
SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION
u~ 0.415
8
0207
C 400 500 600 700
Wavelength, nm
Fig. I. Effect of increasing amounts of triethylamine (0.00, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009 and O.010M respec- tively) on the electronic absorption spectrum of uranyl ion (O.01M) in sulfuric acid (0.1M)
in the visible region makes the uranyl ion a labile
species, undergoing photochemical reduction to urani-
um(IV) (I = 650 nm) in the presence of triethylamine max
-1 (Fig. 2). A strong infrared band at 1705 cm with a
shoulder at 1750 cm -I due to the carbonyl stretching
frequency of acetic acid and ethyl acetate extracted
with ether after a 3 h irradiation of the solution con-
taining uranyl ion (0.04M) at 20• ~ Photochemical
oxidation of triethylamine with uranyl ion is similar
378
SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION
0.130 u t - O
0
<
0.06- c
j "1,~,'--'=V" - ' . ~ ~" 400 500 600 700
Wavetength , nm
Fig. 2. Electronic absorption spectra of photolyzed solutions containing UO~ + (0.01M), H + (0.1M) ions and triethylamine ~0.01M) at time inter- vals of two minutes
to that observe with chlorine dioxide 14'15 The cation +
formed (CH3-CH=N(C2H5) 2) in aqueous solution yields sec-
ondary amine and acetaldehyde, which is photochemically 2,3
oxidized to acetic acid in the presence of uranyl ion
+ +
CH3-CH = N(C2H5) 2 + H20 ----> CH3CHO + H2N(C2H5) 2 (I)
The luminescence intensity of uranyl ion was monitored
in acetone-water medium with excitation at i = 345 nm and
379
SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION
22
20 // ~r
Tmr ie tN~tYrloQan~ ~[ en e - ~ ' ~ / / 18- p-Nitroanilin],~ / / z x /
o " >/ i
Concentration ,xl0 -3
F i g . 3 . S t e r n - V o l m e r p l o t s f o r q u e n c h i n g u r a n y l i o n l u m i n e s c e n c e e m i s s i o n a t 5 0 6 nm w i t h X e x c i t a - t i o n = 345 nm
emission at I = 506 nm. The Stern-Volmer equation (Eq. 2)
If/If = I + kq T[Q] = I + Ksv[Q ] (2)
is applicable only at low concentration of triethylamine.
However, at higher concentration of triethylamine, a pos-
itive deviation from linearity (Fig. 3) rules out the
simple collisional deactivation of excited uranyl ion,
ground state charge transfer complex formation competing
with dynamic deactivation of uranyl iron by nitrogen
donor species (Scheme I). Aromatic z-electrons, play a 16 significant role in quenching uranyl ion luminescence
1 1 UO 2 + :N(C2Hs) 3 ~ ~.uv 2 -~-UO +~ H20+
+CH3-- CH = N(C2 H5 )2 (1 -5)hv a hvf Shy
K o ! +
UO 2*+ :N(C2Hs) 3 ~ (UO~ :N(C2Hs) 3)
380
SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION
The relatively low value of the Stern-Volmer constant
for p-nitroaniline (Table I) may be due to its resonance
stabilization, where a positive charge is delocalized
on nitrogen atom in p-position but not in m-position to
the nitro group.
Here kf, kq and k_1 are the radiative decay, biomole-
cular diffusion and back diffusion rate constants, re-
spectively. K is the equilibrium constant for ground o
state complexation. Triethylamine is partitioned between
ground state and excited state of uranyl iron, forming a
ground state complex and exciplex, respectively. By vir-
tue of their position, the excited state complex is prone
to undergo immediate electron transfer from amine to
urany! ion.
Uranium(V) disproportionates to uranium(IV) and ura-
nium(VI) as soon as it in formed photochemically in
aqueous acidic medium 12'17
In summary, at low concentration of triethylamine,
preferencial protonation of the amine prevents it from
static complexation. However, at its higher concentra-
tion, static quenching plays a significant role in ad-
dition to dynamic annihilation of uranyl ions because
of hard-hard interaction and, consequently, uranyl ion
is reduced to uranium(IV) through an electron transfer
mechanism, followed by disproportionation of uranium(v)
in aqueous medium.
Financial assistance by CSIR, New Delhi (Scheme No.
5/157/89-EMR II) a~d research facilities to PV KB) by
Guru Nanak Dev University are gratefully acknowledged.
381
SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION
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